Disease Update: Watch Out For Diseases during Apple Bloom

Control for fire blight and apple scab is critical during bloom. (Photo: K. Peter)

No rest for the weary these days! Based on the constant hum of the sprayers over the last couple of weeks, I know folks have been proactive protecting their trees. This week will be a good challenge: blossoms are open and we're nearing peak dispersal of the overwintering apple scab spores. It's raining while I'm writing this, so if folks didn't get a chance to get an antibiotic/fungicide spray on this weekend, you will definitely want to get out later today. Below are some nuggets of wisdom to kick off your week:

Fire blight

Infection conditions April 17

Depending on location, many folks are experiencing some kind of apple bloom right now. Open blossoms mean vulnerability for fire blight.

We're in the midst of fire blight conditions in the southern half of PA, so growers will want to be sure their blossoms are protected.

Let's review the minimum requirements for blossom infection and the order in which they must occur:

Flowers must be open with petals intact (flowers in petal fall are resistant)

An accumulation of at least 198 degree hours above 65°F

A wetting as rain or even as dew

An average daily temperature of 60°F

Keeping these criteria in mind, here's a refresher for your approach for managing fire blight:

Antibiotics only work when the blooms are open. Apply antibiotics as complete sprays and add an adjuvant or surfactant.

Antibiotic sprays are most effective when they are applied the day before or the day after an infection event (within 24 hrs!).

Streptomycin is still the best option since it kills the bacteria and has partial systemic activity. Streptomycin still works in the Mid-Atlantic. We have not detected streptomycin resistance in the Erwinia amylovora population in PA; however, still follow the four application limit of strep during bloom (re: the label). Note: the systemic activity does not persist like fungicides and you have about a 48 hour window.

If you have young, dwarf trees that are a susceptible variety (i.e., Fuji, Gala, etc.): growers may want to consider incorporating Actigard in two of their strep sprays. Actigard induces the plant immune response and it is labelled to be used as a tank mix with streptomycin. In our evaluations, Actigard works best on small, young trees compared to larger, semi-dwarf trees. We have the most experience to date with Actigard; however, we are evaluating several similar products this season.

If you have older, susceptible semi-dwarf trees: Prohexadione calcium (ProCa; Apogee or Kudos) will be your best bet to limit the shoot blight phase of fire blight post bloom. Since ProCa takes 10-14 days to kick in, applying this product around the last streptomycin spray or at petal fall is recommended. ProCa will harden off the shoots, which will make the shoots not susceptible to shoot blight.

If conditions post bloom continue to be favorable for spread of the disease, we have observed shoot blight being limited by applications of the copper, Cueva (1 qt/A) and Cueva (1 qt/A) + Double Nickel (Bacilllus amyloliquifaciens; 2 qt/A). As with any copper, copper can cause fruit russetting. Be mindful of conditions and varieties where post bloom copper sprays are used, especially when conditions are slow drying. To limit shoot blight, growers would want to start at petal fall and apply every 10-14 days until terminal bud set (when the shoots are no longer susceptible to infection). We have observed favorable results on dwarf and semi-dwarf trees. We have the most experience with Cueva for fire blight control; however, we're evaluating additional copper products this season.

Do Not spray antibiotics post petal fall. A trauma event (hail, high winds) is the exception.

For antibiotic alternatives, unfortunately, options are limited. We have seen some control (30-40%) using Blossom Protect. However, there is significant fruit russet potential when wet periods follow bloom. Lime sulfur is still the main choice for many folks choosing to not use strep; however, be careful about timing and rate since lime sulfur can thin blooms.

Final thoughts

The cooler days and nights might mean a protracted bloom, which will translate into a bloom that will feel like it will never end (think back to 2014). There is also the issue of rattail bloom. All blossoms are susceptible to infection if the bacteria and conditions are present. As the bloom season shakes out, stay tuned for additional guidance…

Apple scab

Use FRAC Group 7 fungicides late pink through petal fall

April 17 is an infection period for apple scab. According to the models for southern PA, we've had four scab infection periods in the last three weeks (including today): March 26- 29; April 4; April 6. Rain is in the forecast over the coming week, as well. This is important considering we're nearing the peak of mature overwintering spores being dispersed. Consequently, this spells significant disease pressure.

As I had recommended during the winter meetings, from late pink until petal fall is your time to use the FRAC Group 7 fungicides (SDHI class).

During this critical time, growers should consider applying a total of 2 complete sprays (4 half sprays) of products containing FRAC Group 7:

Aprovia (FRAC Group 7)

Fontelis (FRAC Group 7)

Luna Tranquility (FRAC Groups 7 and 9)

Luna Sensation (FRAC Groups 7 and 11)

Merivon (FRAC Groups 7 and 11)

Sercadis (FRAC Group 7)

Important considerations to keep in mind

Please practice fungicide resistance management! The label for products containing FRAC Group 7 states a maximum of 4 (complete) applications per season, and no more than 2 applications in a row. Regardless if the FRAC Group 7 product is a standalone (Aprovia, Fontelis, Sercadis) or in a premix (Lunas, Merivon), ALL count toward your allotment of FRAC Group 7 sprays for the season.

I strongly encourage growers to save 2 of those FRAC Group 7 sprays for the end of the season in order to apply Merivon and/or Luna Sensation before harvest. These products will help with rots showing up in storage. Rots showing up months after harvest in storage are becoming a real headache for a lot of folks.

Always tank mix with a broad spectrum (EBDC, Captan) and rotate with another FRAC group. Other fungicides to consider rotating with a FRAC Group 7 could be Indar (FRAC Group 3), Inspire Super (FRAC Groups 3 and 9), Vangard (FRAC Group 9), or Scala (FRAC Group 9).

Using a strobilurin class (FRAC Group 11) standalone product (Flint, Sovran) is cautioned during this peak time of disease pressure considering there is evidence of fungicide resistance to this class of chemistry in Pennsylvania. The premix FRAC Group 11 products are recommended, such as Merivon or Luna Sensation.

Powdery Mildew

Beware of the dry periods

Powdery mildew is considered a "dry weather" disease. The fungus does not like prolonged leaf wetness (i.e., apple scab conditions); high humidity and temperatures ranging from 55 - 70°F are enough for the spores to germinate. Be mindful of dry weather and protect your trees. Trees are most susceptible from tight cluster until the shoots harden off (approximately second cover spray). The powdery mildew fungus likes young tissue, which is why powdery mildew prone varieties should be protected through terminal bud set.

Even if dry conditions persist during late pink through petal fall, the FRAC Group 7 products are also excellent for powdery mildew control during this time period.

Rust

The galls are active

This is just a friendly reminder the rust galls are active. The products applied right now to combat apple scab will also manage rust, especially if an EBDC is included in the tank mix. At this time, a comparable alternative to managing rust is still elusive (that I'm aware of).

For a rust review:

Each rust spot on your apple leaf is caused by one rust spore coming from the galls on the cedar trees.

Once infection is established on the leaves/fruitlets on the apple tree, the spores produced from those infections will not continue to infect the plant tissue on the apple tree. The spores developed later in the season on that infected plant tissue will fly out and land on the cedar tree and infect it, thereby establishing the cycle all over again.

Severe rust infections on the leaves can defoliate the tree, as well as infect and deform fruit.

Rust management typically occurs from when the galls appear until when a dry period persists after petal fall. The moral of the story: once you see rust spots on your apple tree, it's too late for protection. Hence the need to get protection on early to prevent the disease.